Forestry machines for cutting, accumulating, processing and harvesting cut trees are well known in forestry operations. Typically, a forestry implement is mounted at the end of a boom or crane which has its other end mounted to a turntable so as to be pivotable about a lateral axis relative to the turntable. The turntable is mounted to a turntable support so as to be pivotable about a vertical axis (when the support is vertical). A carriage, which can have tracks or wheels for conveying the turntable support, has a frame to which the turntable support is mounted by a leveling mechanism. The leveling mechanism is operable to vary the position and orientation of the turntable support relative to the carriage, depending upon the slope of the terrain and the orientation of the carriage on the terrain. For example, if trees are being cut, harvested or processed on a hillside, the machine will typically be driven onto the hillside so the trees are within reach of the boom and the leveling mechanism adjusted to make the axis of the turntable generally vertical. The center of gravity of the swing-house assembly, which includes the boom (with cutting head attached), the engine of the machine, and the operator cab, which are mounted to the turntable, should be positioned relative to the carriage so that the machine does not tip forward or backward when the tree is cut or picked up/handled and its weight comes to rest on the butt plate of the head.
Such a mechanism may be made using linkages and hydraulic cylinders for moving linkages as described in U.S. Pat. No. 4,679,803, which is incorporated by reference as if fully set forth herein. In that mechanism, one set of links is provided forwardly and another set is provided rearwardly, and hydraulic cylinders connect the links both laterally and longitudinally to tilt the turntable relative to the carriage both about a lateral axis and a longitudinal axis. Other mechanisms for positioning and orienting the swing-house assembly are also discussed in U.S. Pat. No. 4,679,803.
U.S. Pat. No. 6,173,973, which is incorporated by reference as if fully set forth herein, is directed to a swing-house leveling mechanism for a forestry machine. The leveling mechanism is arranged between the swing-house assembly and the carriage. The leveling mechanism includes a front link and rear link that is shorter than the front link. Simultaneous extensions/retractions of hydraulic cylinders cause the turntable support of the swing-house assembly to move and tilt longitudinally, about a lateral axis relative to the carriage. Differential extensions/retractions of the cylinders cause the turntable support to tilt sideways. The forestry machine provides for the turntable to be rotatable relative to the turntable support about the turntable axis by 360° so that the boom of the machine can be operated in front of the carriage, in back of the carriage, or to either side at any angular position.
Leveling mechanisms must prevent the turntable support from contacting the undercarriage and must prevent damaging contact within the leveling mechanism itself. This can be achieved by using shortened hydraulic cylinders that don't allow the undercarriage and turntable support to contact in any cylinder orientation. Short cylinders, however, limit the operating envelope of the leveling mechanism.
Limit switches and/or cylinder stops can also be used to prevent damaging contact when using longer cylinders. For example, a limit switch can be employed to limit roll angle while allowing for long cylinders that provide greater pitch angles. However, limit switches abruptly cut off cylinder movement and create an operating envelope that is smaller than the operating envelope defined by actual interference/contact between the undercarriage and the turntable support.
The operator of a forestry machine normally controls a leveling mechanism via four command buttons for the directions fore, aft, left, and right, which directions apply to when the cab is directed forwardly relative to the carriage. The command buttons are usually logically connected to the leveling hydraulic cylinders so that the operator has to be aware of the orientation of the cab to the undercarriage. Operating the leveling mechanism on a slope can be especially difficult for even the most experienced operators because the cab where the operator sits may be rotated 360° in relation to the undercarriage.